1. Field of the Invention
The present invention relates to a variable pipette which is designed to have a variable volumetric capacity for sucking and discharging samples.
2. Description of the Related Art
As one example of a conventional variable pipette, an "Adjustable Pipette" is disclosed in Japanese Patent Publication No. 57-13338 (1982). According to this prior art, a continuous plunger unit (having a first first-stage stop portion and a first second-stage stop portion) is axially and slidably accommodated in a body. This plunger unit is biased rearwardly by a first-stage spring so that the first first-stage top portion is brought into contact with a second first-stage stop portion which is movably provided on the body. Further, a member which defines a second second-stage stop portion is movably fitted on the plunger unit and axially and slidably accommodated in the body. This second second-stage stop member is similarly biased rearwardly by a second-stage spring so as to be brought into contact with a first-stage spring retainer which is formed as an integral part of the body. In operation, the plunger unit is first pushed against the force of the first-stage spring so as to slide by an amount corresponding to a first-stage stroke until the first and second second-stage stop portions abut against each other. In operation, the tip portion at the distal end of the pipette is immersed in a sample. Subsequently, the plunger unit is allowed to slide back, thus causing the sample to be sucked up into the tip portion. Then, the plunger unit is pushed so as to slide by an amount corresponding to the first-stage stroke again, thereby discharging the sample. Thereafter, the plunger unit, together with the second second-stage stop member, is pushed so as to slide by an amount corresponding to a second-stage stroke against the forces of the first- and second-stage springs, thus completely discharging the sample remaining in the tip.
According to the above-described prior art, the first-stage stroke is set between the plunger unit and the second second-stage stop member through the second first-stage stop portion, the body and the first-stage spring retainer. Since the body of the pipette is gripped in the operator's hand, the temperature of the body rises and inevitably thermal expansion is caused. Therefore, if the first-stage stroke is set through the body as in the case of the above-described arrangement, an erroneous stroke may be produced during the use of the pipette. Accordingly, the prior art involves the risk that a problem may arise from the thermal expansion in regard to the reproducibility of quantitative determination carried out when a sample is sucked and discharged. Since the accuracy of this reproducibility is usually questionable to the extent of something like 0.3 to 0.4% of the suction capacity of the pipette, the effect of the above-described thermal expansion of the body cannot be ignored, and it has therefore been desired to solve the problem of this thermal expansion. In addition, since the parts which define the first-stage stroke are assembled on the basis of the body, it is not only difficult to conduct the assembly operation itself but also to lower the cost of assembly.